The field of vision of a motor vehicle window, in particular a windshield, must be kept free of ice and condensation. In the case of motor vehicles with an internal combustion engine, a stream of air heated by engine heat can, for example, be directed to the windows.
Alternatively, the window can have an electrical heating function. Thus, composite glass panes that have a transparent, electrically conductive coating on an interior-side surface of one of the individual panes are known. Using an external voltage source, an electric current that heats the coating and, with it, the pane can be conducted through the electrically conductive coating. WO2012/052315 A1 discloses, for example, such a heatable, electrically conductive coating based on metal.
The electrical contacting of the electric heating layer is typically done via busbars, as is known from US 2007/0020465 A1. The busbars are made, for example, from a printed and fired silver paste. The busbars typically run along the upper and lower edge of the pane. The busbars collect the current that flows through the electrically conductive coating and conduct it to external feed lines that are connected to a voltage source.
The patterning of the electric heating layer by separating lines or separating zones for the formation of a usually winding current path is known from the industrial series production of panes with electrical heating layers. This has the advantage that the electrical resistance can be increased and the current path can be contacted by relatively small connection electrodes. In the patent literature, such a surface heater is described, for example, in the German published patent application DE 19860870 A1.
Panes having an electric heating layer shield relatively strongly against electromagnetic radiation such that, in particular in motor vehicles with a heatable windshield, radio data traffic can be significantly impaired. Heatable windshields are, consequently, frequently provided with uncoated and thus coating-free zones (“communication windows”). The uncoated zones are readily permeable at least for specific ranges of the electromagnetic spectrum and, in this manner, enable trouble-free data traffic through the pane. The uncoated zones, on which electronic devices, such as sensors, cameras, and the like, are situated, are often arranged in the vicinity of the upper edge of the pane where they can be readily concealed by the upper masking strip.
However, uncoated zones negatively affect the electrical properties of the heating layer, which at least locally affects the current density distribution of the heating current flowing through the heating layer. In fact, they cause a highly inhomogeneous heating power distribution, with the heating power significantly reduced below and in the vicinity of the uncoated zones. On the other hand, spots with a particularly high current density develop, in which the heating power is greatly increased. As a consequence, very high local pane temperatures which constitute a risk of burns can occur and impose high thermal stresses on the panes. In addition, this can cause loosening of bonding sites of attachments.